Devices made from single-layer transition metal dichalcogenides (TMDs) offer the promise of inexpensive, flexible, high-performance electronics that exploit their unique monolayer and surface-dominated geometry. Abbreviated chemically as MX2, where M is a transition metal (Mo, W, Nb, etc.) and X is a chalcogen (S, Se, or Te), the TMDs behave as insulators, semiconductors, metals, magnets, and superconductors with a variety of properties distinct from bulk. For instance, the semiconductors MoX2 and WX2 transition from indirect gap semiconductors in the bulk to direct gap as monolayers.
Chemical vapor sensing with monolayers is a particularly promising field, as their inherent few-atom thinness results in extreme sensitivity to surface perturbations. MoS2 is an extraordinarily sensitive chemical vapor sensor, responding selectively to strong electron donors (e.g. amines) through a physisorption process.
A minute quantity of analyte on the surface of the MoS2 acts as an electron donor and local reducing agent, measurably affecting the conductance of the channel. Analytes relevant to identifying explosives and nerve agents have been detected to concentrations as low as 10-50 parts per billion (ppb) by monitoring the conductance of a simple MoS2 field effect transistor (FET). Such sensitivity is comparable to the current state-of-the-art conductance, surface acoustic wave (SAW), and optical chemical vapor sensors.
TMD sensors are also flexible, inexpensive, robust, and require only nanoamperes for operation, making them intrinsically ultra-low power, distinct advantages over other types of sensors.
As physisorption is strongly dependent on the band structure of the material, a variety of TMDs can be combined into a single sensing suite to identify compounds of interest, analyze mixtures, and add sensitivity to the devices, building in essence a synthetic nose.
Most TMD devices used Au only or Ti/Au as the contact metal, which resulted in electronic behavior that is entirely dominated by Schottky contacts. For chemical vapor sensing in particular, poor contacts strongly limit the current in the channel and produce uncontrollable and variable sensitivity to polar molecules. Some suggest that a low work-function material, such as Sc, or a tunable work function material, such as graphene, can provide better contacts. However, Sc only lowers the Schottky barrier without completely eliminating it, and using graphene adds an additional level of complexity to the devices. To achieve the highest functionality, it is suggested that the contacts be Ohmic and not Schottky-limited.